Just wondering, if I have a model which needs to be split into two parts (such as for allowing some sections to be printed in another material), how do I make it so that there's a button that lets people buy both?

I've seen some models (mainly multi-piece twisty puzzles) with a link on the page that lets you buy, for instance, 1xPartA + 26xPartB (where PartA and PartB would be uploaded separately, I presume). That's the sort of link I want to create, I just can't seem to find a button that lets me do that.